Color is highly subjective yet it can be submitted to scientific measurement. We all use our eyes which, coupled with our brain, prove to be the most efficient color comparison computer. Human eyes as a computer, are self-lubricating and structurally efficient. They consume mineral energy, are maintained at a constant temperature and react to all properties of color. However, eyes are not infallible. Each person sees color differently and there is no precise prismatic color memory. Accordingly, color is subjective. Human eyes measure all characteristics of color at once but scientific measurements supplement the eye. These systems are concerned with only one property of color at any given measurement.
Color separation processes have never been standardized and probably never will be because of the need of customized separations, either to compensate for the different printing processes or to emphasize an effect in a supplied original of a particular product. Over the years, many masking and separation systems have been developed, some are simple and others are very elaborate. Naturally the more elaborate systems involve more elaborate processes. Additionally, the systems become more expensive as additional labor and material is employed. Even with the more complex systems, there is no single system that will provide the optimum tone reproduction.
Technology has advanced to a point where it is possible to produce separations in a fraction of the time with a high degree of quality and flexibility. With electronic scanning, the color separation process has become more and more automated and technology continues to increase. But even with the technology now available, the basis of the difficulty concerning all separators, conventional or electronically operated, is still the determination of optimum tone reproduction. Percent tone reproduction is very important.
Color correction is important but is still secondary to good gray balance. Color correction may be influenced by the inclination of energy absorption of the color separation filters, the spectral range of the photo multipliers and the position of the logarithmic circuits. These type of parameters control the addition and subtraction of components and exponents. However, certain color hues can be corrected mechanically later in the design of the product as long as the picture content and resolution has the shape and tonal reproduction.
Spectral energy output is a method of classification of originals and a determination of their tone reproduction to maximize reproduction efficiency. This objectively based method would enable a scanner operator to select a tone reproduction curve from such a system for that class of an original. The system could be expected to efficiently produce pleasing color reproduction from a wide range of transparencies without human intervention. Studies to find the optimum tone reproduction curve have involved the making of a large number of reproductions which varied in their tonal content. Experiments have found that tone reproduction curves are potentially more accurate because they are based on the picture contact rather than on a gray scale and basic standard aim points. Utilizing statistical sampling of the tones of the picture, and curves derived from the originals can be used to objectively classify the picture by type, processing and contrast.
Data for generating the tone reproduction curves can be obtained by measuring the density of the originals and the density of the gray scale scanned along with the reproduction. The gray scale is used as a stand-in for the picture because it is more convenient to measure. Important information to the overall concept of reproduction includes the computation of the characteristic of the densitometer, press and the mode of modular transfer functions. Classification was considered by comparing and ranking originals by their dye-sets to define their frequency of occurrence for any range of tones. Since contrast can range in the highlight, midtone and shadows of an original, it is useful to numerically measure each of these areas. That type of information can be derived by a slight variation of the Jones-type diagram. This charge diagram was used to divide the tone reproduction curves into four quadrants,* using the curves of the reproduction and the original. This criteria enables a computation to distinguish the curve slopes and, hence, the image classification without human intervention. Therefore, the cumulative frequency of the quadrants show a tabulation into a percent density reference valve corresponding to the highlight, midtone and shadow. The overall curve shapes show the slopes to be lateral linear. Holding the highlight point of the picture slope of the curve maintains the color cast and signifies the magnitude. The corresponding tone encountered at the lower end depicts the picture saturation. This difference multiplied by a percent density reference number signifies the density for the midtone placement, hence, the optimum tone reproduction. In this manner, a picture can be thought of as consisting of a large number of single areas having different tones and lightness. For example, in high key pictures, a large percentage of the tonal areas are light while the largest percentage of tones in a low key picture are dark. With this in mind, originals can be objectively classified not only by their dye-sets but also by contrast and type. The optimum tone reproduction curve is the relationship between what can be considered more negative or more positive with respect to the gray line. This distribution is derived directly from the tonal gradation range content of each dye layer of the original. FNT *1. DENSITY RANGE OF THE ORIGINAL FNT 2. DENSITY RANGE OF COMPRESSION FNT 3. PERCENT DOT REPRODUCTION OF THE DENSITY SLOPE FNT 4. NEUTRAL INK BALANCE OF THE TONE REPRODUCTION
The theory of spectral energy output is not only based on the linear characteristic curves and logarithmic balance needed for printing ink but also takes into consideration the spectral energy dye-set of the original.
The spectral energy output is derived from the original with the use of a three-filter spectral densitometer. This derives information from the original on the basis of its dye-set absorption and provides a numerical determination to coordinate the position of the midtone gradation automatically. The red, green and blue filter readings are required to define the dye-sets of the original in order to determine the curve slope to render a balanced neutral output.
Different color materials are designed to meet different objectives and have different dye-sets for different jobs. There are colors which may appear the same to the eye but which have different spectral energy distribution outputs. In this theory, the densitometer measures an exact match for an exactly defined color. The tone reference number value corresponds to the general shape of the tone reproduction curve. Technically, each printing ink pigment should absorb 1/3 of the visible spectrum. However, both the yellow and magenta pigments absorb in the blue portion of the spectrum and thus, the pigments are not ideal. By this mutual absorption, a precise tone reproduction relationship exists between the yellow, magenta and cyan process inks in order to render a neutral gray at the output.
A neutral balance product can be derived from the original's own energy output at any given tone reference number for a shop's standard. A reproduction can require a color shift either because of a bad cast in the original or because of a particularly bad ink or press condition or even to custom tailor a desired special result.
The output of the product can be modified intentionally by using two or more different tone reference numbers either in the direction of dark or light and the resultant curve will either be convex or concave.
In taking into consideration the spectrum energy outputs of the original it should be kept in mind that the visible white light spectrum absorption is 400-700 nanometers, that is the light reflected or transmitted as seen by the eyes. Light is energy wave motion starting with red as the least energy and progressively more energy through orange, yellow, green, blue, indigo, and violet. The colors having the greatest absorption are those with the least energy output.
There is a geometrical logarithmic progression, that is the density of the common logarithm in relation to the reciprocal of transmittance. It should also be kept in mind that there is a linear relationship involved in that the reproduction output values are directly proportional to the original output densities.
There are colors which may appear the same to the eye but which have different spectrum energy distribution outputs. With the theory of spectral energy output, the densitometer measures a density match for an exactly defined color. For the color balance, a dye-set target is exposed and developed in such a way that the colors would equal the optimum of its own dye-set nature in magnitude and saturation and are used to calibrate the color correction.
It would certainly be advantageous to develop a quick and efficient calculator device and method to take advantage of the theory of spectral energy output as set forth above.
Calculators have been used in many environments to take advantage of a given arrangement for theoretical facts to simplify and assist in quickly and efficiently applying the theory to a given practical approach in a particular environment.
For example, U.S. Pat. No. 3,719,806 shows a slide rule type device used in calculating halftone screen exposures. A plurality of reciprocally movable slide members with appropriate linear scales, a base, a cursor, and predetermined curves on the structure for alignment with the linear scales are part of the design.
Similar calculating devices are used in a variety of different environments as can be seen in U.S. Pat. Nos. 1,881,165; 2,434,306; 2,569,454; 2,746,682; 2,793,808; 2,960,267; 3,024,977; 3,135,465; 3,162,363; 3,522,655; 3,572,583; 3,652,831; 4,071,189; 4,146,173; 4,179,610; and 4,186,297. However none of these references shows a structure which is adaptable for use as a mid-tone calculator for determining optimum density reproduction in a lithographic tone process.